Michael Bay decided to do a fourth 'Transformers' movie because he didn't want anyone else to ''screw it up''.

The 47-year-old director - who helmed the first three films in the franchise - previously said he wouldn't do another motion picture in the series, but he was inspired to take on the challenge upon seeing the success of the 'Transformers' rollercoaster ride at Universal Studios, Hollywood when it opened in May.

He said: ''I thought I was done. Then the ride came out and the two-and-a-half-hour lines. And then you're thinking, 'Oh my God, someone's going to take this over.' And you start doing a lot of soul-searching.

''Like, OK, I'm about to do a little movie, 'Pain & Gain' ... and the studio says they want to restart the franchise. And someone could come in here and screw it up, you know? So I'm thinking that if I do this last one, we set it on a new footing, we change a lot of things - but we keep the history of the three in place.

''But we broaden it so it can be set up and be carried on - it would have a better chance for survival, I guess. You know?''

Michael confirmed Peter Cullen will reprise his role as Optimus Prime in 'Transformers 4', and he revealed the storyline for the forthcoming film will ''carry on'' from last year's third instalment of the franchise, 'Transformers: Dark of the Moon', rather than be a reboot.

Speaking to The Huffington Post, he added: ''We're basically taking from the history of where it was - or where we left it in Chicago. And we're going to carry it on from there.''